We and others have found that there are unique DNA synthesizing enzymes in precursors of lymphoid cells that may serve as tumor markers. A group of enzymes (TdT, ADA and PNP) have been selected as potential markers of lymphoid precursors. We have emphasized the development of immunological tools to detect these enzymes in small quantities of cells. It is our opinion that these techniques will ultimately be extraordinarily useful in clinical management of patients with leukemia and lymphoma.